This invention relates to the liquid phase epitaxy (LPE) growth of epitaxial layers on Inp substrates.
A major problem encountered in LPE on InP has been the thermal instabilitY of the substrate at growth temperatures. Severe thermal decomposition, resulting in microscopic pitting of the substrate surface, may be largely eliminated by protecting the substrate with excess phosphorus vapor produced, for example, by an indium phosphide cover piece, an elemental phosphorus source, a phosphine source, or an indium-tin-phosphorus solution. However, even on a protected, pit-free substrate, some photoluminescence and defect studies have revealed the existence of a thermally-degraded surface, which must be removed to achieve high quality epitaxial layers and good device performance.
In LPE practice, a clean, undegraded surface with uniform and consistent wetting is commonly produced by melting back the substrate with pure indium just prior to the first epitaxial growth, as described by V. Wrick et al, Electronic Letters, vol. 12, p. 394 (1976). A pure indium melt, however, attacks the substrate rapidly and unevenly; the resulting surface of the substrate is not flat but rippled. Surface rippling for a (100)-InP substrate increases with the depth of meltback and also increases with the size of the substrate. For example, when LPE growth is scaled to large (.about.15.times.17 mm) wafers, surface rippling is a particularly severe problem.
In addition to making subsequent device processing easier, a flat surface is also critical for the performance of many devices. As an example, consider double heterostructure edge-emitting LEDs and lasers. The typical structure comprises a guaternary InGaAsP active layer (.about.0.2 .mu.m thick) sandwiched between n-InP and p-InP cladding layers. In the ideal (flat surface) case, the emitted light is transmitted along a straight waveguide. If the substrate initially is not flat, however, the subsequent epitaxial layers are also not flat. In this case, the waveguide is not straight, and the surface ripples strongly scatter the light.
Addition of phosphorus to indium has been used for a more controlled meltback as described by K. E. Brown, Journal of Crystal Growth, Vol. 20, p. 161 (1973). Although a nearly saturated In/P melt produces flat surfaces, nonuniform wetting often occurs. Consequently, growth is spotty and may not cover the entire substrate surface.
In more complicated devices, such as a buried heterostructure requiring LPE regrowth over a mesa, increased contamination due to additional processing steps (e.g., etching to form the mesa), in addition to the nonideal geometry (e.g., vertical side walls of the mesa), makes clean wipe-off difficult. After the usual In meltback, In-rich beads cling to the mesa. These beads solidify into In-rich inclusions which are sources of dislocations and which degrade device performance. Achieving a clean wipe-off requires removal of essentially all contaminants from the substrate surface. In the usual, single In meltback, contaminants which are not fully dissolved may settle back onto the mesas, leading to incomplete wipe-off.